The present invention relates to a semiconductor memory device such as a SRAM and an electronic instrument using the same.
In a semiconductor memory device having a memory cell such as a static random access memory (SRAM) and a dynamic random access memory (DRAM), increasing both a packing density and a speed is always a very important object. Then, in this kind of semiconductor memory device, the number of memory cells in the direction along a word line and in the direction along a bit line increases with increasing the packing density.
When the number of memory cells in the direction along the word line increases, the number of memory cells directly connected to one word line increases and a load resistance and a load capacity of the one word line increase, which makes it difficult to selectively drive the word line at a high speed.
Thus, a memory cell array is divided into a plurality of memory blocks in the direction along the word line and a plurality of main word lines are arranged across the plurality of memory blocks. Further, in each of the plurality of memory blocks are arranged a plurality of sub word lines following each of the plurality of main word lines. Reducing the load capacity of the one main word line in this manner enables a high speed driving.
On the other hand, in the semiconductor memory device such as the SRAM, in a period of a state in which data is not read or written but is held, that is, in a period of a stand-by state, the bit line has a high potential such as a power source voltage. However, when the bit line is put into contact with a wiring layer having a low potential such as a ground wiring or the word line by a foreign matter mixed in a manufacturing process, a short-circuit current is passed through the bit line to cause a current failure in the bit line. As a relief measure against the current failure like this, there is a technology in which a fuse is provided between the bit line and a power source terminal and is cut by the use of a laser or the like to enable the bit line causing the current failure to be separated from the power source.
This kind of fuse is formed of a conductive material such as polysilicon, tungsten silicide, or aluminum and in order to enable the fuse to be cut by the use of the laser or the like, it is necessary to arrange the fuses at sufficient wide intervals and to narrow their widths. Thus, a portion where the fuse is arranged has higher resistance as compared with a usual wiring layer. In addition, as described above, since the number of memory cells connected to the respective bit lines increases with increasing the packing density, there is a possibility that initializing to a high logic level necessary for an action of reading data by precharging the bit line before switching a state of selection of the word line can not be performed at a sufficient high speed. This results in reducing an access speed in the semiconductor memory device.